Book/Dissertation / PhD Thesis

FZJ-2017-00269

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Influence of Initial Thermomechanical Treatment on High Temperature Properties of Laves Phase Strengthened Ferritic Steels

Talik, M. (Corresponding author)FZJ*

2016
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-95806-175-0

Please use a persistent id in citations: http://hdl.handle.net/2128/14065

Abstract: The aim of this work was to design 17 wt%Cr Laves phase strengthened HiperFer ($\textbf{Hi}$gh $\textbf{per}$fomrance $\textbf{Fer}$rite) steels and evaluate their properties. This class of steel is supposed to be used in Advanced Ultra Super Critical power plants. Such cycles exhibit higher efficiency and are environmentally friendly, but improved materials with high resistance to fireside/steam oxidation and suficient creep strength are required. The work focused on the characterization of creep properties of 17Cr2.5W0.5Nb0.25Si heat resistant steel. Small batches of steels with nominal compositions of 17Cr3W0.5Nb0.25Si and 17Cr3W0.9Nb0.25Si were used to analyze the inuence of chemical composition on the precipitation behaviour in comparison to 17Cr2.5W0.5Nb0.25Si steel. Creep strength of HiperFer steels is ensured by fine dispersion of thermodynamically stable Laves phase particles, while maintaining high corrosion resistance by a relatively high chromium content. Design of HiperFer steels was accomplished by thermodynamic modeling (Thermocalc) with the main tasks of elimination of the unwelcome brittle (Fe,Cr)-$\sigma$ phase and maximization of the content of the strengthening C14 Fe$_{2}$Nb type Laves phase particles. Long term annealing experiments of all HiperFer steels were performed at 650 °C in order to evaluate the role of chemical composition and initial thermo-mechanical treatment state on precipitation behaviour. Laves phase particles formed quickly after few hours and the size of precipitates did not change signicantly within 1,000 hours. The observed development of Laves phase particles was compared with thermodynamical calculations (TC-Prisma). The creep properties of 17Cr2.5W0.5Nb0.25Si steel in different initial thermo-mechanical treatment states were tested at 650 °C. The influence of different cold rolling procedures, and heat treatments was investigated. Increased cold rolling deformation had a positive effect resulting not only from work hardening, but from the acceleration of Laves phase particle precipitation. The recrystallization annealed material exhibited signficantly shorter creep life in comparis onto the rolled material. Two promising heat treatments restoring the creep strength up to the level of rolled material were identified. Tempering at 540 °C accelerates the formation and reduces the size of strengthening Laves phase precipitates. Short term tempering at 800 °C causes dense precipitation of Laves phase particles at grain boundaries without detrimental influence on the morphology of Laves phase particles in the grain interiors. Dense precipitation of coarse Laves phase particles causes a retardation of grain boundary sliding in the later periods of creep exposure.

Note: RWTH Aachen, Diss., 2016

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Contributing Institute(s):

1. Werkstoffstruktur und -eigenschaften (IEK-2)

Research Program(s):

1. 899 - ohne Topic (POF3-899) (POF3-899)
2. HITEC - Helmholtz Interdisciplinary Doctoral Training in Energy and Climate Research (HITEC) (HITEC-20170406) (HITEC-20170406)

Appears in the scientific report 2017

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